Numerous applications require eliminating electromagnetic radiation reflections. The large number of electronic systems incorporated in vehicles gives rise to an increase of electromagnetic interferences. This problem includes false images, radar interferences and reduced performance due to the coupling between systems. A microwave absorber may be very effective for eliminating this type of problems. There is even greater interest in reducing the echoing area of certain systems to prevent or minimize detection thereof.
Microwave absorbers are made by modifying the dielectric properties or, in other words, the dielectric constant and magnetic permittivity, or magnetic permeability, of certain materials. The first case involves dielectric absorbers which base their operation on the principle of resonance at one-fourth of the wavelength. However, the second case involves the absorption of the magnetic component of the radiation. The first attempts made to eliminate reflections include the Salisbury absorbing screen method, the non-resonant absorber, the resonant absorber and resonant magnetic ferrite absorbers. In the case of the Salisbury screen, a screen with a carefully chosen electric resistance is placed at the point where the electrical field of the wave is maximum, i.e. at a distance equal to one-fourth of the wavelength with regard to the surface to be screened. This method has little practical use since the absorber is too thick and is only effective for too narrow a band of frequencies and variation of incident angles.
In the non-resonant methods, radiation crosses through a dielectric sheet to subsequently be reflected by the metal surface. The dielectric sheet is thick enough so that in the course of its reflection, the wave is sufficiently attenuated before reemerging from the sheet. As the sheet must be made of a material having low losses at high frequency and low reflection properties to assure penetration and reflection, the sheet must be very thick to effectively attenuate the wave.
In the first resonant methods, materials with high dielectric losses are placed directly on the conductive surface to be protected. The dielectric material has an effective thickness, measured inside the material, approximately equal to an even number of one-fourths of semi-wavelengths of the incident radiation. The utility of the method is limited due to the substantial thickness of the dielectric sheet and to the narrow absorption band they have, especially at low frequencies. Attempts have been made to make up for these deficiencies by dispersing ferromagnetic conductive particles in the dielectric material. However, when metal particles, high permeabilities, in the range of 10 or 100, disperse, they are not compatible with low conductivities, in the range of 10−2 or 10−8 mmhos per meter.
Another type of absorbers are those known as ferrite absorbers (see, for example, U.S. Pat. No. 3,938,152), having clear advantages over those already set forth herein. They function in the form of thin sheets such that they overcome the drawbacks of the substantial thickness required by dielectric absorbers. Furthermore, they are effective for frequencies between 10 MHz and 15,000 MHz, and they dissipate more energy than dielectric absorbers do.
Ferrite absorbers developed hitherto eliminate reflections by means of sheets of insulating or semi-conductive ferrites, and particularly ferromagnetic metal oxides, placed directly on the reflective surface. In these cases, the term ferrite refers to ferromagnetic metal oxides including, but not limited to, spinel, garnet, magnetoplumbite and perovskite type compounds.
In this type, the absorption is of two types, which can occur simultaneously or not. These are dielectric and magnetic losses. The first losses are due to the electron transfer between the cations Fe2+ and Fe3+, whereas the ones of the second type originate from the movement and relaxation of spins of the magnetic domains.
According to certain inventions (such as U.S. Pat. No. 3,938,152), at low frequencies, generally those in the range between UHF and the L-band, energy is predominantly extracted from the magnetic component of the incident radiation field, whereas at higher frequencies, generally in the L-band and higher, energy is equally extracted from the electric and magnetic component.
This type of absorbers eliminates reflection because the radiation establishes a maximum magnetic field on the surface of the conductor. In the normal incidence of a flat wave on an ideal conductor, complete reflection occurs, the reflected intensity is equal to the incident intensity. Incident and reflected waves come together, then generating a standing wave in which the electric field is nil at the border of the conductor, whereas the magnetic field at that border is maximum. There is a condensation of the magnetic field for the maximum time possible. In this manner, in the case of ferrite, it is necessary for the incident radiation to go through the absorbing sheet to establish the maximum magnetic field conditions. It has been seen that the complex part of the permeability of certain ferromagnetic metal oxides varies with the frequency such that it enables obtaining low reflections on very broad frequency ranges without needing to use magnetic absorbers of substantial thicknesses as in other cases.
Taking into account the reflection coefficient in metals for normal incidence, it is deduced that when working with a thin sheet, the reflected wave can be attenuated regardless of the electric permittivity of the absorbing material. Minimum reflections will occur at a certain frequency if the complex permeability μ″ is substantially greater than the real one μ′ as long as the product Kτ<<1, where K is the wave number and τ is the thickness of the sheet.
The present invention refers to a type of element susceptible of being used in supports for electromagnetic radiation absorption, known as magnetic microwire.
The known Taylor's technique used for the manufacture of microwires enables obtaining them with small diameters comprised between one and several tens of microns. Microwires thus obtained can be made from a large variety of alloys and magnetic and non-magnetic metals. This technique is disclosed, for example, in the article “The Preparation, Properties and Applications of Some Glass Coated Metal Filaments Prepared by the Taylor-wire Process”, W. Donald et al., Journal of Material Science, 31, 1996, pp. 1139–1148.
The technique for obtaining magnetic microwires with insulating sleeve and amorphous microstructure is disclosed, for example, in the article “Magnetic Properties of Amorphous Fe—P Alloys Containing Ga, Ge and As” H. Wiesner and J. Schneider, Stat. Sol. (a) 26, 71 (1974), Phys. Stat. Sol. (a) 26, 71 (1974).
On the other hand, the determination of the manufacturing conditions so that the microstructure of the metal core of the obtained microwire is amorphous are disclosed in U.S. Pat. No. 5,240,066, wherein the ranges within which certain manufacturing parameters must be comprised are disclosed, such as: the superheating temperature of the melted alloy (250–300° C. higher than the melting temperature of the alloy), the length of the cooling area (5–7 mm), the distance from the cooling area to the heating area (40–50 mm), the cooling rate (105–106 K/s), etc.
The drawback of the control of magnetic properties such as initial magnetic permeability and magnetic anisotropy field of the metal microwire which, being coated with an insulating sleeve, furthermore has an amorphous structure, according to the manufacturing and processing parameters, have been considered previously in Spanish patent ES 2,138,906, referring to a “Method of Manufacture and Processing of Amorphous Metal Microwires Coated with an Insulating sleeve with High Magnetic Properties” In this case, control of the technical parameters necessary for obtaining microwires with a high real part of magnetic permeability is involved.
Properties of amorphous magnetic microwires with an insulating sleeve are also disclosed in the article “Natural Ferromagnetic Resonant in Cast Microwires Covered by Glass Insulation”, A. N. Antonenko, S. A. Baranov, V. S. Larin and A. V. Torkunov, Journal of Materials Science and Engineering A (1997) 248–250.